Coated piston ring for an internal combustion engine

ABSTRACT

A piston ring with a coated outer surface is provided. The coating is disposed on end sections of the outer surface adjacent a gap. Typically, a middle section of the outer surface located between the end sections is not coated. The coating can be formed of CrN or DLC, and the CrN coating can be applied by physical vapor deposition (PVD). The end sections of the outer surface, upon which the coating is applied, are rough. For example, the outer surface can be blasted or otherwise textured to achieve the rough surface. The rough surface retains oil and distributes stress better than a smooth surface, and thus reduces crazing and flaking of the coating.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to a piston for an internalcombustion engine, a piston ring for providing a seal between the pistonand a cylinder wall of the engine, and methods of manufacturing thesame.

2. Related Art

Typical internal combustion engines are provided with at least onepiston assembly which reciprocates within a cylinder of an engine block.In general, each piston assembly includes a piston body with a pluralityof ring grooves, each of which typically receives and operably supportsa piston ring. In operation, the piston rings remain in the ring groovesand travel with their respective piston bodies in a reciprocating motionwithin cylinders of an engine block. Among other things, the pistonsrings function to seal combustion gasses in a combustion chamber abovethe piston body, to transfer heat from the piston body to the cylinderwall, to restrict the passage of oil from the crank case to thecombustion chamber and to provide a generally uniform oil film on thecylinder wall. Piston rings are oftentimes coated to improve performanceand/or reduce wear and damage to the piston ring. However, over time,due to the harsh conditions of the engine, the coating can experienceflaking and crazing.

SUMMARY

One aspect of the present invention provides a coated piston ring for apiston assembly of an internal combustion engine which is less prone toflaking and crazing. The piston ring comprises a ring body extendingcircumferentially around a center axis between opposite end surfaces,and the end surfaces of the ring body present a gap therebetween. Thering body has an outer surface facing away from the center axis, theouter surface includes end sections adjacent the end surfaces, and theend sections of the outer surface are rough. A coating is disposed onthe end sections of the outer surface, and the coating includes chromiumnitride (CrN) or diamond-like carbon (DLC).

Another aspect of the invention provides a piston assembly for aninternal combustion engine. The piston assembly includes a piston bodyincluding at least one ring groove extending circumferentially around acenter axis, and a ring body disposed in one of the at least one ringgroove. The ring body extends circumferentially around the center axisbetween opposite end surfaces, and the end surfaces of the ring bodypresent a gap therebetween. The ring body has an outer surface facingaway from the center axis, the outer surface includes end sectionsadjacent the end surfaces, and the end sections of the outer surface arerough. A coating is disposed on the end sections of the outer surface,and the coating includes chromium nitride (CrN) or diamond-like carbon(DLC).

Yet another aspect of the invention provides a method of manufacturing apiston ring. The method comprises the steps of providing a ring bodyextending circumferentially around a center axis between opposite endsurfaces, wherein the end surfaces present a gap therebetween, and thering body has an outer surface facing away from the center axis. Themethod further includes texturing end sections of the outer surfacelocated adjacent the end surfaces so that the end sections of the outersurface are rough; and applying a coating to the rough end sections ofthe outer surface, wherein the coating includes chromium nitride (CrN)or diamond-like carbon (DLC).

Another aspect of the invention provides a method of manufacturing apiston assembly. The method comprises providing a piston body includingat least one ring groove extending circumferentially around a centeraxis, and providing a ring body extending circumferentially around thecenter axis between opposite end surfaces. The end surfaces of the ringbody present a gap therebetween, and the ring body has an outer surfacefacing away from the center axis. The method further includes texturingend sections of the outer surface adjacent the end surfaces so that theend sections of the outer surface are rough, applying a coating to therough end sections of the outer surface, wherein the coating includeschromium nitride (CrN) or diamond-like carbon (DLC), and disposing thecoated piston ring in one of the at least one ring groove of the pistonbody.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bereadily appreciated, as the same becomes better understood by referenceto the following detailed description when considered in connection withthe accompanying drawings wherein:

FIG. 1 is a perspective view of a piston ring according to an exemplaryembodiment;

FIG. 2 is an enlarged cross-sectional view of the piston ring of FIG. 1;and

FIG. 3 is a sectional view of an internal combustion engine including apiston assembly with the piston ring of FIG. 1 according to an exemplaryembodiment.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

An exemplary embodiment of a piston ring 20 for an internal combustionengine is shown in FIG. 1. A coating 22 is disposed on rough sections ofan outer surface 24 of the piston ring 20. Due to the rough outersurface 24 beneath the coating 22, the coating 22 less prone to crazingand flaking than comparative coatings on piston rings without the roughsurface beneath the coating.

The piston ring 20 comprises a ring body extending circumferentiallyaround a center axis A between opposite end surfaces 26. The ring bodyis formed of metal, for example steel. The end surfaces 26 of the ringbody present a gap 28 therebetween. Typically, the end surfaces 26 areparallel to the center axis A, but the end surfaces 26 could be disposedat an angle relative to the center axis A. The outer surface 24 of thering body faces away from the center axis A, and the ring body includesan inner surface 30 facing toward the center axis A. A top surface 32extends from the inner surface 30 to the outer surface 24 of the ringbody, and a bottom surface 34 is spaced from the top surface 32 andextends from the inner surface 30 to the outer surface 24. Typically,the inner surface 30 is parallel to the center axis A and the outersurface 24, and the top surface 32 is perpendicular to the center axis Aand parallel to the bottom surface 34, but the surfaces could bedisposed at different angles relative to one another.

The outer surface 24 of the ring body includes end sections 36 adjacentthe end surfaces 26 and the gap 28. The end sections 36 of the outersurface 24 are rough, and the coating 22 is disposed on at least therough end sections 36 of the outer surface 24. The end sections 36 arespaced from one another by a middle section 38 of the outer surface 24.In the example embodiment shown in FIG. 1, the middle section 38 is notcoated with the coating 22. In this embodiment, the end sections 36 ofthe outer surface 24 have a surface roughness greater than a surfaceroughness of the middle section 38, and the middle section 38 of theouter surface 24 is considered to be smooth. However, in anotherembodiment, the entire outer surface 24, including the middle section38, is coated with the coating 22. The rough sections of the outersurface 24, also referred to as textured surfaces, distribute stressesdifferently than smooth surfaces in a way that reduces flaking andcrazing of the coating 22. The rough sections of the outer surface 24also create a surface area that retains more oil on the piston ring 20.

The coating 22 applied to the end sections 36, and possibly to theentire outer surface 24, includes chromium nitride (CrN) or diamond-likecarbon (DLC). The coating 22 may consist essentially of, or consistentirely of DLC or CrN. The coating 22 is formed of grains 46, and dueto the rough surface, the grains 46 of the coating 22 are disposed atangles relative to one another.

Another aspect of the invention provides a method of manufacturing thecoated piston ring 20. The method includes texturing the end sections 36of the outer surface 24 located adjacent the end surfaces 26 of the ringbody so that the end sections 36 of the outer surface 24 are rough; andapplying the coating 22 to the rough end sections 36 of the outersurface 24. The texturing step can include blasting the outer surface 24with a solid material, such as sand, to form the rough surface.

The coating step typically includes physical vapor deposition,especially when the coating 22 includes CrN, but other methods can beused to apply the coating 22. In the example embodiment, the middlesection 38 includes no coating 22, so the method includes applying noneof the coating 22 to the middle section 38 of the outer surface 24. Forexample, the method can include masking the middle section 38 of theouter surface 24 while applying the coating 22 to the end sections 36,so that the coating 22 is only disposed on the end sections 36. However,the coating 22 could be disposed on the entire outer surface 24. Themethod can also include masking the middle section 38 of the outersurface 24 while blasting or texturing the outer surface 24 in anothermanner, so that only the end sections 36 are textured. Alternately, theentire outer surface 24 could be rough, with or without the coating 22on the middle section 38.

The coated piston ring 20 is preferably disposed on a piston body toform a piston assembly 40 for an internal combustion engine. The pistonbody can have various different designs, but typically includes a crown42 and a skirt 44 depending from the crown 42. The piston body typicallyincludes at least one ring groove extending circumferentially around thecenter axis A, and the piston ring 20 is disposed in one of the ringgrooves. In the example embodiment, the crown 42 of the piston bodyincludes a plurality of the ring grooves and at least one, andoftentimes more or all of the ring grooves, contain the piston rings 20.

Many modifications and variations of the present invention are possiblein light of the above teachings and may be practiced otherwise than asspecifically described while within the scope of the claims. It is alsocontemplated that all features of all claims and of all embodiments canbe combined with each other, so long as such combinations would notcontradict one another.

What is claimed is:
 1. A piston ring, comprising: a ring body extendingcircumferentially around a center axis between opposite end surfaces,said end surfaces of said ring body presenting a gap therebetween, saidring body having an outer surface facing away from said center axis,said outer surface including end sections adjacent said end surfaces,said end sections of said outer surface being rough, a coating disposedon said end sections of said outer surface, and said coating includingchromium nitride (CrN) or diamond-like carbon (DLC).
 2. The piston ringof claim 1, wherein said coating consists essentially of DLC or CrN. 3.The piston ring of claim 1, wherein end sections are spaced from oneanother by a middle section of said outer surface, and said middlesection is not coated with said coating including CrN or DLC.
 4. Thepiston of claim 1, wherein said end sections have a surface roughnessgreater than a surface roughness of said middle section of said outersurface.
 5. The piston ring of claim 1, wherein said coating includes aplurality of grains disposed at angles relative to one another.
 6. Thepiston ring of claim 1, wherein said coating includes CrN.
 7. The pistonring of claim 6, wherein said coating is applied by physical vapordeposition.
 8. The piston ring of claim 1, wherein said coating includesDLC.
 9. The piston ring of claim 1, wherein said end surfaces of saidring body are parallel to said center axis, said ring body has an innersurface facing toward said center axis, said ring body has a top surfaceextending from said inner surface to said outer surface and a bottomsurface spaced from said top surface and extending from said innersurface to said outer surface, said inner surface is parallel to saidcenter axis and said outer surface, said top surface is perpendicular tosaid center axis and parallel to said bottom surface, said ring body isformed of steel, said end sections are spaced from one another by amiddle section of said outer surface, said middle section of said outersurface is not coated with said coating including CrN or DLC. said endsections of said outer surface have a surface roughness which is greaterthan a surface roughness of said middle section, said coating consistsessentially of DLC or CrN, and said coating includes a plurality ofgrains disposed at angles relative to one another.
 10. A piston assemblyfor an internal combustion engine, comprising: a piston body includingat least one ring groove extending circumferentially around a centeraxis, a ring body disposed in one of said at least one ring groove, saidring body extending circumferentially around the center axis betweenopposite end surfaces, said end surfaces of said ring body presenting agap therebetween, said ring body having an outer surface facing awayfrom said center axis, said outer surface including end sectionsadjacent said end surfaces, said end sections of said outer surfacebeing rough, a coating disposed on said end sections of said outersurface, and said coating including chromium nitride (CrN) ordiamond-like carbon (DLC).
 11. The piston assembly of claim 10, whereinsaid piston body includes a crown and a skirt depending from said crown,and said at least one ring groove is located in said crown.
 12. A methodof manufacturing a piston ring comprising the steps of: providing a ringbody extending circumferentially around a center axis between oppositeend surfaces, the end surfaces presenting a gap therebetween, the ringbody having an outer surface facing away from the center axis; texturingend sections of the outer surface located adjacent the end surfaces sothat the end sections of the outer surface are rough; and applying acoating to the rough end sections of the outer surface, the coatingincluding chromium nitride (CrN) or diamond-like carbon (DLC).
 13. Themethod of claim 12 including applying the coating by physical vapordeposition.
 14. The method of claim 13, wherein the coating includesCrN.
 15. The method of claim 12, wherein the coating includes DLC. 16.The method of claim 12, wherein the end sections are spaced from oneanother by a middle section of the outer surface, and applying none ofthe coating to the middle section of the outer surface.
 17. The methodof claim 16, wherein the texturing step provides the end sections of theouter surface with a surface roughness which is greater than a surfaceroughness of the middle section.
 18. The method of claim 12, wherein theend surfaces of the ring body are parallel to the center axis, the ringbody has an inner surface facing toward the center axis, the ring bodyhas a top surface extending from the inner surface to the outer surfaceand a bottom surface spaced from the top surface and extending from theinner surface to the outer surface, the inner surface is parallel to thecenter axis and the outer surface, the top surface is perpendicular tothe center axis and parallel to the bottom surface, the ring body isformed of steel, the end sections are spaced from one another by amiddle section of the outer surface which is not textured, the endsections have a surface roughness which is greater than a surfaceroughness of the middle section, the coating consists essentially of DLCor CrN, and the coating includes a plurality of grains disposed atangles relative to one another being applied to the outer surface; andapplying none of the coating to the middle section of the outer surface.19. A method of manufacturing a piston assembly comprising the steps of:providing a piston body including at least one ring groove extendingcircumferentially around a center axis, providing a ring body extendingcircumferentially around the center axis between opposite end surfaces,the end surfaces presenting a gap therebetween, the ring body having anouter surface facing away from the center axis, texturing end sectionsof the outer surface adjacent the end surfaces so that the end sectionsof the outer surface are rough, applying a coating to the rough endsections of the outer surface, the coating including chromium nitride(CrN) or diamond-like carbon (DLC), and disposing the coated piston ringin one of the at least one ring groove of the piston body.
 20. Themethod of claim 19, wherein the end sections are spaced from one anotherby a middle section of the outer surface, and applying none of thecoating to the middle section of the outer surface.